Resonant mode active matrix TFEL display excitation driver with sinusoidal low power illumination input

ABSTRACT

Energizing an active matrix electroluminescent device with a generally sinusoidal illumination waveform causes the electroluminescent layer to emit light. A sinusoidal waveform minimizes the peak currents reducing the likelihood of burnouts and decreases the imposed voltages on the data lines increasing the likelihood that the high voltage transistors will function as intended. Preferably, the sinusoidal waveform is generated by using a single 12 volt power source which reduces the expense, weight and bulk of the electroluminescent device. The 12 volt power source may be used to operate an operational amplifier that receives a small sinusoidal input signal and produces a low voltage generally sinusoidal waveform that is amplified by a step-up transformer for energizing the transparent electrode layer of the device so as to cause the electroluminescent layer to emit light. Furthermore, the use of a generally low voltage operational amplifier permits the routing of a 12 volt power signal from a remote power source, such as a battery, to a head-mounted active matrix electroluminescent device, reducing safety concerns routing of high voltage signals near the body of the user.

BACKGROUND OF THE INVENTION

The present invention relates to a generally sinusoidal resonant modeexcitation driver circuit for an active matrix electroluminescentdisplay.

Conventional row drivers for passive thin-film electroluminescentdisplays employ both positive and negative high voltage direct-currentpower supplies and discrete integrated switching devices. Low voltagelogic pulses are applied to the row driver inputs to switch the rowdrivers outputs between the positive and negative high voltage powersupplies. Switching between a pair of power supplies produces positiveand negative generally rectangular shaped waveforms, e.g., pulse-typewaveform, that are imposed across the electroluminescent phosphor layerof the display. An example of a similar type of drive network is Flegal,U.S. Pat. No. 4,733,228, assigned to the same assignee. The componentsrequired to fabricate, or otherwise construct, each of the high voltagepower supplies may be expensive, heavy, and bulky. Moderately sizedpower supplies may be acceptable for desktop sized passive thin-filmelectroluminescent displays where space is available.

The row drivers for passive thin-film electroluminescent displays aredigital devices designed to operate by switching between a pair ofopposing polarity direct-current power supplies to produce pulsedgenerally rectangular waveforms. The rectangular waveform has a fastrise time for positive voltages and a fast fall time for negativevoltages.

An active matrix electroluminescent (AMEL) device is constructed of athin-film electroluminescent stack fabricated on a rearwardly disposedsubstrate layer, typically made of silicon. A portion of the substratelayer includes a circuit layer to select individual pixels within theelectroluminescent stack. Energizing a transparent electrode layerwithin the electroluminescent stack causes all selected pixels withinthe device to simultaneously emit light.

Active matrix thin-film electroluminescent devices are frequently usedas head-mounted displays, so the bulk and weight associated with twohigh voltage power supplies located near the head of the user is notacceptable. Further, AMEL devices are frequently portable batterypowered devices and the power consumption associated with two highvoltage power supplies could require a larger battery. Additionally, itis difficult to accurately control the magnitude of the output signalwhen switching between two direct-current power supplies, which in turnreduces the ability to generate a high quality gray scale image on thedisplay. Pulse-type waveforms also induce high peak currents that couldresult in burnouts and degraded pixel controllability within the activematrix thin-film display rendering some portions of the displaynonfunctional.

Stewart, U.S. Pat. No. 5,302,966 discloses a circuit element to selectan individual pixel of an active matrix electroluminescent display. Inthe Stewart device, the source of a high voltage transistor is connectedto a data line. The drain of the high voltage transistor is connected toan electrode of the electroluminescent layer. Energizing the transparentelectrode layer illuminates selected pixels within the display with thedata lines functioning as a ground return. Data lines have resistanceand therefore, the greater the current carried by the data lines, thegreater the voltage drop over the length of the data lines. Pulse typewaveforms induce high peak currents which impose significant voltagedrops over the length of the data lines. These imposed voltage drops mayreach such a high level that the sources of the high voltage transistorswill develop a high voltage which may interfere with the gate drive ofthe high voltage transistors.

What is desirable, therefore, is a driver for an active matrixelectroluminescent display that does not require a pair of high voltagepower supplies. Elimination of these power supplies reduces the expense,weight, and bulk of the device. Also, the device should minimize theimposed voltage drops on the data lines, reduce burnouts, and provideaccurate control over the magnitude of the applied illumination waveformto the transparent electrode layer.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks of theprior art with a driver circuit and a method of energizing an activematrix electroluminescent device with a generally sinusoidalillumination waveform so as to cause the electroluminescent layer toemit light. A sinusoidal waveform minimizes the peak currents reducingthe likelihood of burnouts and decreases the imposed voltages on thedata lines thereby avoiding imposing a large voltage on the sources ofthe high voltage transistors which improves pixel controllability andbrightness.

For active matrix electroluminescent devices it is desirable to replacethe generally rectangular row waveform with a generally sinusoidalillumination waveform. The sinusoidal waveform minimizes the peakcurrents thereby reducing the voltage imposed along the data lines, thusresulting in higher likelihood of proper operation of the high voltagetransistors. Additionally, the sinusoidal waveform reduces the overallstress on the device resulting in fewer burnouts. The reduction inburnouts also increases the life expectancy of the display and reducesthe number of field returns.

However, conventional wisdom is that the electronic circuitry requiredto produce a sinusoidal 400 volts peak-to-peak waveform with accuratecontrol of the waveform voltage is difficult and expensive withintegrated circuit devices, as currently employed with passive thin-filmelectroluminescent displays. In contrast to traditional wisdom, thepresent invention includes a driver for a sinusoidal illuminationwaveform that permits accurate control over the output voltage and isrelatively inexpensive.

Preferably, the sinusoidal waveform is generated by using a single 12volt power source which reduces the expense, weight and bulk of theelectroluminescent device. The 12 volt power source is used to power anoperational amplifier that receives a low voltage sinusoidal inputsignal and produces an amplified, generally sinusoidal voltage waveformthat is further amplified by a step-up transformer for energizing thetransparent electrode layer of the device so as to cause theelectroluminescent layer to emit light. This method eliminates the useof a pair of expensive, heavy, and bulky high voltage power suppliestypically used for passive thin-film electroluminescent devices.

Furthermore, the use of a generally low voltage operational amplifierpermits the routing of a 12 volt power signal from a remote powersource, such as a battery, to a head-mounted active matrixelectroluminescent device. This reduces safety concerns associated withrouting high voltage signals near the body of the user.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an active matrix electroluminescentdevice.

FIG. 2 is an exemplary electrical schematic of a driver circuitconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an active matrix electroluminescent (AMEL) deviceis constructed of a thin-film electroluminescent (laminar) stackcomprising a transparent front electrode 170 carrying an illuminationsignal (waveform), which is typically indium tin oxide deposited on atransparent substrate 182 (glass). A transparent electroluminescentphosphor layer 174 is sandwiched between front and rear dielectriclayers 176 and 178, all of which are deposited behind the frontelectrodes 170. Alternatively, either the front or rear dielectric layer176 and 178 may be omitted. Pixel electrodes 186a, 186b, 186c, and 186dare deposited on the rear dielectric layer 178, typically consisting ofa pad of metal or poly-silicon, positioned at each location a pixel isdesired within the phosphor layer 174. A first isolation layer 188,second isolation layer 190, and ground plane 192 are deposited on thepixel electrodes 186a-186d and exposed rear dielectric layer 178. Thefirst and second isolation layers 188 and 190 are preferably constructedout of SiO₂ or glass. The first and second isolation layers 188 and 190,and ground plane 192 are preferably constructed with holes, commonlyreferred to as VIA, for each pixel electrode 186a-186d, to permit theconnection of the pixel electrodes to a circuit layer 172 which isdeposited on a substrate layer 180. The substrate layer 180 is typicallysilicon. The circuit layer 172 permits the individual addressing of eachpixel electrode 186a-186d by its associated circuit element 184a-184d.As such, an individual pixel within the electroluminescent layer 174 maybe selectively illuminated by the circuit layer 172 permitting asufficient electrical field to be created between the front electrode170 and the respective pixel electrode 186a-186d. The circuit layer 172and circuit elements 184a-184d therein may be any suitable design, suchas those disclosed in U.S. patent application Ser. No. 08/293,144,assigned to the same assignee and incorporated herein by reference.

Referring to FIG. 2, an oscillator (not shown) provides a low voltagegenerally sinusoidal input signal 12 to the driver circuit 10. Anysuitable source may be used that produces a generally or substantiallysinusoidal signal. A capacitor C1 electrically decouples any DCcomponents of the input signal 12 from the remainder of the drivercircuit 10. Electrically decoupling the direct current component of theinput signal 12 allows the driver circuit 10 to independently set thedirect-current bias point for amplification of the input signal 12. Apair of series resistors R1 and R2 are connected between a 12 voltdirect-current power supply 14 and ground 16 to select thedirect-current bias point, which is preferably approximately six volts,for the non-inverting input 17 of an operational amplifier 26.Decoupling the input signal 12 and selecting a direct-current bias pointfor the input signal 12 to an operational amplifier 26 requires only asingle 12 volt power supply and permits the use of a low power, smallamplitude, input signal 12 which can be readily generated by a simpleoscillator circuit. The operational amplifier could alternatively usedual low voltage power supplies or a higher voltage power supply, ifdesired.

The six volt direct-current bias point, which is half of the voltagefrom the power supply 14 applied to the operational amplifier 26,permits the maximum voltage swing from the output 18 of the operationalamplifier 26 without clipping. The input signal 12 and signal from thepower supply 14 are preferably transmitted from a remote location to thedisplay, such as the user's belt-worn computer, when the display is usedas a head-mounted device. The transmission of relatively low voltagesfrom the power supply 14 and input signal 12 minimizes safety concernswith routing high voltage signals near the body of the user when thedisplay is used as a head-mounted device. An example of such ahead-mounted system is described in U.S. Patent Application entitled"Substrate Carriers for Electroluminescent Displays," filed Jun. 23,1995, assigned to the same assignee and incorporated herein byreference.

The output 18 of the operational amplifier 26 is connected to theinverting input 22 of the operational amplifier 26 through a network,including a parallel capacitor C3 and resistor R4 network. The capacitorC3 attenuates high frequency signals to prevent oscillations. Theresistor R4 controls a portion of the gain of the output 18 of theoperational amplifier 26. Alternatively, resistor R4 could be connectedin parallel with a variable resistor (not shown) to permit variable gaincontrol, or otherwise replaced with a variable resistor to permitvariable gain control. Increasing and decreasing the resistance of R4increases and decreases, respectively, the gain of the output 18.Additionally, a resistor R3 is connected between the inverting input 22of the operational amplifier 26 and a capacitor C2 to provide furthergain control. Increasing and decreasing the resistance of R3 decreasesand increases, respectively, the gain of the output 18. Resistor R3 maybe replaced by a variable resistance. The resistor R3 is capacitivelycoupled to ground 16 through capacitor C2 to eliminate anydirect-current gain which helps prevent clipping of the output 18.

Connected to the power supply input 28 of the operational amplifier 26is a sufficiently large capacitor C5 to store energy for peak currentdemands and a bypass capacitor C6 to attenuate high frequency signals toprevent oscillation. The combination of the capacitors C5 and C6 providedecoupling for the power supply 14.

The output 18 of the operational amplifier 26 is capacitively coupled bycapacitor C4 to the remainder of the driver circuit 10. The capacitor C4prevents driving direct-current signals into a low impedance primary 18of a step-up-transformer 20. This reduces the power required from theoutput 18 of the operational amplifier 26.

The primary 22 of the step-up-transformer 20 receives a generallysinusoidal signal from the output 18 of the operational amplifier 26.The windings of the primary 22 and secondary 24 are selected to increasethe magnitude of the sinusoidal signal to preferably a 400 voltpeak-to-peak illumination waveform. When the input signal 12 is firstenergized the slew rate of the operational amplifier 26 limits the rateof increase of its output 18. However, the rate of increase may besufficiently high, producing peak currents that result in burnouts andimpose a sufficiently large voltage drop on the data lines. The resistorR5 limits the peak current levels produced when initially imposing theinput signal 12 at the inverting input 17 of the operational amplifier26.

The driver circuit 10 provides a 400 volt peak-to-peak generally orsubstantially sinusoidal waveform. Additionally, this circuit operatingwith a limited number of relatively inexpensive components. The drivercircuit 10 may operate either continuously or in a burst mode to providethe desired illumination waveform with the desired magnitudes byadjusting the gain of the operational amplifier 26. The gain may beadjusted at any appropriate time. Accordingly, only single low voltagepower supply, such as a 12 volt power supply, is required which reducesthe bulk, size, and expense of the driver circuit.

The secondary 24 of the transformer 20 is generally modeled as aninductor. The display 30 is generally modeled as a capacitive load basedupon the weighted average of a typical display pattern (i.e., theaverage number of illuminated pixels). The frequency of the appliedsinusoidal waveform is preferably selected at the resonant frequency ofthe tank circuit formed by the combined transformer secondary 24 and thedisplay 30 so as to minimize the dissipation of energy. The preferredfrequency of the illumination waveform is about 5 KHz. The inductance(L) of the secondary 24 is selected to be L=1/ 4π² CF² !.

The combination of the inductance of the secondary 24 and capacitance ofthe display 30 provide a resonant system at the desired frequency whichminimizes the power required to operate the display. Theoretically,after the system is at resonance, there is no reactive powerconsumption. However, some real power is consumed by the inherentresistance of the electronic components and display. Minimizing thepower consumption is important for a battery powered active matrixelectroluminescent device.

It is also important to ensure that the rest of the system does notoscillate at resonance. The resistor R5 also moves the primary 22 offresonance so that the remainder of the system does not oscillate.

To determine the proper inductance of the primary 22, the inductance ofthe secondary 24 is divided by the turns ratio squared. With the turnsratio and the desired magnitude of the illumination waveform, thenecessary voltage from the output 18 of the operational amplifier 26 isdetermined. The gain of the operational amplifier 26 is adjusted toprovide the desired magnitude of the illumination waveform.

For monochrome and color displays a gray scale is desirable in order todisplay video and graphic images with better screen clarity anddefinition. Many current techniques to achieve a gray scale for thinfilm electroluminescent displays can be broadly categorized as thosecalling for modulation of the amount of charge flow through the phosphorlayer. Those techniques may be further divided into two subcategories,namely, amplitude modulation and pulse width modulation. A furthermodulation technique is to employ bursts of pulses of variable timeduration applied to the phosphor layer, such as those described in U.S.patent application Ser. No. 08/341,404, assigned to the same assigneeand is incorporated herein by reference. To achieve a gray scale usingseveral of the available techniques, the transparent electrode driverrequires accurate amplitude control and the ability to provide multiplebursts of pulses.

The driver circuit 10 has the ability to operate in a burst mode. Theinput signal 12 provides the desired number of sinewave pulses and theoutput of the transformer 20, in turn, supplies the correspondingillumination signal. The resistor R5 limits the rate of increase of thesignal so that current levels are maintained at acceptable levels.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

What is claimed is:
 1. A method of illuminating an electroluminescentdevice comprising the steps of:(a) providing said electroluminescentdevice for said illuminating with a plurality of layers including atleast a transparent electrode layer, a circuit layer, and at least twolayers including an electroluminescent layer and a dielectric layer,said at least two layers disposed between said circuit layer and saidtransparent electrode layer; (b) receiving a low voltage generallysinusoidal input waveform and in response producing a generallysinusoidal intermediate waveform having a voltage greater than saidinput waveform; (c) receiving said intermediate waveform in a primary ofa step-up transformer and in response generating a generally sinusoidalillumination waveform having a voltage greater than said intermediatewaveform at a secondary of said transformer; and (d) energizing saidtransparent electrode layer with said illumination waveform so as tocause said electroluminescent layer to emit light.
 2. The method ofclaim 1 further comprising the step of selecting a frequency of saidillumination waveform to be near a resonant frequency of a circuitcomprising the combination of an inductor formed by said secondary and acapacitor formed by said plurality of layers.
 3. The method of claim 2further comprising the step of controlling an impedance of said primarysuch that said primary does not resonate with said combination of saidsecondary and said plurality of layers.
 4. The method of claim 2 furthercomprising the step of receiving said input waveform in an operationalamplifier and producing said intermediate waveform from said operationalamplifier.
 5. The method of claim 4 further comprising the step ofadjusting a gain of said operational amplifier.
 6. The method of claim 4further comprising the step of powering said operational amplifier witha power signal in the range of 12 volts.
 7. The method of claim 6further comprising the step of transmitting said power signal from aremote location to said operational amplifier.
 8. A driver circuit forproviding a generally sinusoidal illumination signal to an active matrixthin-film electroluminescent display comprising:(a) an amplifier havingan input and an output; (b) said amplifier receiving at said input a lowvoltage generally sinusoidal input waveform and producing at said outputa generally sinusoidal intermediate waveform having a voltage greaterthan said input waveform; and (c) a step-up transformer having both aprimary electrically connected to said output so as to receive saidintermediate waveform, and a secondary electrically connected to saiddisplay so as to provide said generally sinusoidal illumination signalhaving a voltage greater than said intermediate waveform.
 9. The drivercircuit of claim 8 wherein said illumination signal is a generallysinusoidal signal of around 400 volts peak to peak.
 10. The drivercircuit of claim 8 wherein said amplifier is powered by an approximately12 volt signal.
 11. The driver circuit of claim 8 further comprising avariable resistor electrically connected to said amplifier to adjust themagnitude of said intermediate waveform.
 12. The driver circuit of claim8 wherein a frequency of said illumination signal is selected to be neara resonant frequency of a tank circuit comprising the combination of aninductor formed by said secondary and a capacitor formed by saiddisplay.
 13. The driver circuit of claim 12 further comprising anelectrical device electrically connected to said primary to select aresonant frequency of said primary to be different than said resonantfrequency of said tank circuit.
 14. The driver circuit of claim 13wherein said electrical device is a resistor.